Compressor-expander set critical speed avoidance

ABSTRACT

A control method and apparatus for critical rotational speed avoidance in a compressor-expander set in a gas refrigeration system. By varying an opening of an antisurge or recycle valve, a shaft power used by the compressor in the compressor-expander set may be varied, thereby varying the rotational speed of the compressor-expander set to move it away from its critical speed zone. Additionally, a feedforward signal may be provided by a compressor-expander set control system to cause an antisurge valve for a recycle compressor to open upon a trip or shutdown of one compressor-expander set.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. Ser. No. 12/047,938 filed Mar.13, 2008, now U.S. Pat. No. 8,360,744 issued Jan. 29, 2013, entitledCOMPRESSOR-EXPANDER SET CRITICAL SPEED AVOIDANCE and is herebyincorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a control scheme. Moreparticularly the present invention relates to a method and apparatus forlimiting a time of operation in a critical speed zone of turbomachinery.This invention also relates to an antisurge scheme for a recyclecompressor when a compressor-expander set trips.

2. Background Art

Most turbomachines, such as compressors, gas turbines, steam turbines,and expanders inherently exhibit at least one critical speed where therotational speed of the turbomachine excites a natural frequency of theturbomachine. Extended operation at such critical speeds mustnecessarily be avoided.

When the critical speed or speeds reside below the normal operatingregion of the turbomachine, a startup procedure involving high angularaccelerations through the critical speed or speeds is carried out, thusminimizing the time of operation in a neighborhood of the critical speedor speeds. This neighborhood around a critical angular or rotationalspeed is known as a “critical rotational speed zone,” and is thusdefined for the purposes of this application, including the claims.Critical speed zones for a particular turbomachine are disclosed by theturbomachine manufacturer.

Critical speed zones residing within the normal operating speed range ofa particular turbomachine are less common than those residing outsidethis normal operating speed range.

An improved cryogenic process for liquefying natural gas is disclosed inU.S. Pat. No. 6,308,531 by Roberts et al., and is hereby incorporated inits entirety by reference. The process is also described in a paperpresented at the 2007 LNG 14 conference. The title of this paper is“Technical Challenges during the Engineering Phases of the Qatargas IILarge LNG Trains” by Chavez et al., which is also hereby incorporated inits entirety by reference. The improved process includes a gasrefrigeration cycle using nitrogen for a refrigerant. As is well knownto those of ordinary skill in this art, a gas refrigeration cycle makesuse of a compressor and an expander or turbine. The expander is used todrop the pressure of the gas, but also serves to extract energy from therefrigerant via shaft power. Shaft power derived from the expander isused to provide at least a portion of the required refrigerantcompressor power. Gas refrigeration cycles are covered in manyundergraduate thermodynamics textbooks such as Fundamentals ofEngineering Thermodynamics 6^(th) ed. by Moran and Shapiro, John Wiley &Sons, Inc., publishers, ISBN-13: 978-0471-78735-8 which is herebyincorporated in its entirety by reference.

The gas refrigeration cycle used for producing Liquid Natural Gas (LNG)in the Roberts et al. process is a regenerative cycle. That is, a heatexchanger is used to cool the high pressure stream upstream of theexpander using the relatively cold low pressure stream downstream of thecooling load.

A departure from text-book gas refrigeration cycles in the Roberts etal. LNG application is the use of a first compressor, driven by theexpander, and a second compressor driven by a separate driver. Becauseof the energy provided by the second compressor to the gas stream, theexpander produces sufficient power to fully drive the first compressor.

The gas refrigeration cycle of the Roberts et al. LNG process is thecoldest of a plurality of cascaded refrigeration cycles. Hence, the gasrefrigeration cycle is used to subcool the liquid natural gas below itssaturation temperature.

Typically, a plurality of gas refrigeration cycles, arranged inparallel, is used in the LNG process. The compressors in thecompressor-expander sets may be operated using a load-sharing algorithmsuch as those disclosed in U.S. Pat. No. 5,743,715 to Staroselsky etal., which is hereby incorporated in its entirety by reference.

Turbocompressors generally experience unstable operation at low flowrates. The instability takes the form of either stall or surge, withsurge being the most common for industrial compressors. In surge, theflow through the compressor suddenly reverses direction. This results inlarge thrust loads that can damage thrust bearings and cause vanes tocontact the compressor shroud. Relatively hot gases from the dischargeside of the compressor are drawn back into the compressor where moreenergy is added from the rotor, increasing the gas temperature evenmore. Repeated surge is to be avoided. Surge control algorithms aredescribed in the Compressor Controls Series 5 Antisurge ControlApplication Manual . . . Publication UM5411 rev. 2.8.0 December 2007,herein incorporated in its entirety by reference.

A control system for the refrigeration processes in the Roberts et al.LNG process is needed. A challenging aspect for this control system isavoidance of critical speeds for the compressor-expander sets used inthe gas refrigeration loop. These compressor-expander sets typicallyhave a plurality of critical speed zones, some of which reside withinthe normal operating speed range of the compressor-expander sets.Extended operation in these critical speed zones must be avoided, butthe gas refrigeration process must not be disrupted.

When a compressor-expander set trips or is shut down for any reason,including that of residing too long in a critical rotational speed zone,the second compressor, driven by a separate driver, may be pushed towardsurge.

There is, therefore, a need for an improved control system for acompressor-expander set.

BRIEF SUMMARY OF THE INVENTION

A purpose of the present invention is to provide a method and apparatusfor avoiding operating a compressor-expander set in a critical speedzone longer than a predetermined time while maintaining the process atits set point or set points.

The following description assumes a gas refrigeration cycle wherein therefrigerant is nitrogen, a representative process in which acompressor-expander set is used. The instant control method andapparatus is by no means limited to gas refrigeration cycles or to aparticular fluid used in the system.

Most often, a plurality of compressor-expander sets is provided toproduce the nitrogen mass flow rate needed to subcool the natural gasfeed stock.

The expander in each compressor-expander set is constructed withvariable geometry, often adjustable nozzles. The position of theadjustable nozzles is the manipulated variable used to maintain thetotal nitrogen mass flow rate at a desired set point. The rotational, orangular, speed of the compressor-expander set varies based on therefrigeration load.

While the adjustable nozzles of the plurality of expanders arecollectively manipulated to maintain the total mass flow rate, eachindividual compressor-expander set may be operated at a rotational speedproviding a desired operating condition of the compressor. Inparticular, it is usually undesirable for one compressor to be operatingon its surge control line while another compressor is operating awayfrom its surge control line. It is almost always less efficient for onecompressor to require recycle to avoid surge when another is notrecycling compared to increasing the low-flow compressor's flow rate sono recycle is needed while decreasing the high-flow compressor's flowrate as long as that reduction does not result in recycle.

Therefore, the instant invention calls for manipulation of theexpanders' adjustable nozzles to cause the operating points of thecompressors to reach their surge control lines simultaneously. Oncerecycle begins, recycle flow rate or recycle valve opening may be usedto balance the operation of the compressors.

The compressor in each compressor-expander set is outfitted with arecycle or antisurge valve. When the recycle valve is opened, gas ispermitted to travel from the high pressure discharge side of thecompressor to the low pressure suction side through the valve, thusincreasing the flow rate through the compressor. The recycle valve isused as the manipulated variable by an antisurge control system to avoidoperation in the compressor's unstable surge region.

For most centrifugal compressors over most of the operating range,increased flow rate corresponds to increased power required to drive thecompressor. Accordingly, when the recycle valve is opened, the powerneeded to drive the compressors increases. Even when increased flow rateresults in reduced power requirement, such as is common for axialcompressors, opening the recycle valve results in a change (a decrease)in power required. Therefore, the recycle valve can be used topredictably vary the rotational speed of the compressor-expander set,even while the expander mass flow rate is maintained at a constantvalue.

A novel use of the compressor's recycle valve is that of critical speedzone avoidance. If a compressor-expander set enters one of its criticalspeed zones, the automatic control system will open the compressor'srecycle valve to increase the compressor's flow rate, usually slowingthe rotational speed of the compressor-expander set out of its criticalspeed zone.

An additional object of this invention is the use of a feed-forwardsignal to signal the nitrogen recycle compressor's control system of acompressor-expander set trip.

That is, if a compressor-expander goes into shutdown expectedly orunexpectedly, the nitrogen recycle compressor may be driven towardsurge. By signaling the nitrogen recycle compressor's control system toopen the nitrogen recycle compressor's recycle valve, surge of thenitrogen recycle compressor can be avoided.

The novel features which are believed to be characteristic of thisinvention, both as to its organization and method of operation togetherwith further objectives and advantages thereto, will be betterunderstood from the following description considered in connection withthe accompanying drawings in which a presently preferred embodiment ofthe invention is illustrated by way of example. It is to be expresslyunderstood however, that the drawings are for the purpose ofillustration and description only and not intended as a definition ofthe limits of the invention.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a simplified schematic of a portion of a LNG refrigerationsystem;

FIG. 2 a is a detail schematic of the gas refrigeration cycle;

FIG. 2 b shows a temperature-entropy diagram for the gas refrigerationcycle;

FIG. 3 is a detail schematic of the gas refrigeration cycle withinstrumentation used in the LNG process;

FIG. 4 is a schematic of a LNG refrigeration system comprising aplurality of gas refrigeration compressor-expander sets, eachcommunicating with its own heat exchanger;

FIG. 5 is a schematic of a LNG refrigeration system comprising aplurality of gas refrigeration compressor-expander sets, allcommunicating with a common heat exchanger;

FIG. 6 a is a flow diagram representing a mass flow control system;

FIG. 6 b is a flow diagram representing load sharing and load balancingcontrol;

FIG. 7 a is a flow diagram for critical speed avoidance;

FIG. 7 b is a detail flow diagram of a shutdown sequence;

FIG. 8 is a schematic of a preferred gas refrigeration system forsubcooling liquid natural gas;

FIG. 9 is a compressor performance map for the main compressor inpressure ratio versus flow coordinates;

FIG. 10 is a compressor performance map for the main compressor in powerversus flow coordinates;

FIG. 11 is a compressor performance map for the balance compressor inpressure ratio versus flow coordinates; and

FIG. 12 is a compressor performance map for the balance compressor inpower versus flow coordinates.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, feed stock 100, for this example, natural gas, isfirst dehydrated (not shown) and the heavy components removed (notshown). A relatively high temperature cooling loop 105 (or plurality ofloops) such as a propane refrigeration loop, is used to lower thetemperature of the gaseous feedstock 100 in a high temperature heatexchanger 110.

The feedstock 100 then enters a main heat exchanger 115, where theremainder of the sensible heat is removed, and at least some of thelatent heat is also removed from the feedstock 100. The feedstock 100 isfurther cooled in a subcooling heat exchanger 120 where the temperatureof the feedstock 100 is lowered below the saturation temperature. Atthis point, with very little additional processing, the pressure of theliquid natural gas is dropped and the LNG stream is directed to storage.

The refrigeration loop providing the low temperature stream in the mainheat exchanger 115 is often a mixed refrigerant (MR) refrigerationsystem. Two stages of compression 125 in series, each compression stage125 with its own driver 130 are shown in the schematic of FIG. 1.Typical drivers 130 include gas turbines and steam turbines. Althoughtypical, the depicted arrangement is not universal. Heat is rejected tothe ambient (air or water) by two ambient MR heat exchangers 135.Cooling of the MR to a lower temperature occurs in the high temperatureheat exchanger 110.

The cold MR stream is passed through the main heat exchanger 115 whereheat is transferred from the feedstock 100 stream to the MR stream.

Cascaded with the MR refrigeration loop is at least one gasrefrigeration loop 140. The refrigerant in a typical gas refrigerationloop 140 for LNG production is pure nitrogen (N₂). A nitrogen recyclecompressor 145 is driven by a driver 150, such as a gas turbine. Therotational speed of the nitrogen recycle compressor 145 is typicallyvariable. Referring now to FIGS. 2 a and 2 b, as well as FIG. 1, thenitrogen recycle compressor 145 brings the pressure of the nitrogenrefrigerant from its lowest pressure [state (1)] to an intermediatepressure [state (2)], where heat is rejected from the nitrogen stream tothe ambient in a first nitrogen ambient heat exchanger 155. Thepressure, p, tends to drop from state (2) to state (3), through thefirst nitrogen ambient heat exchanger 155 as seen in FIG. 2 b where thedotted curves represent constant pressure curves. Additionally, thetemperature, T, decreases and entropy, s, is transferred out of thenitrogen stream with the heat.

From the outlet [state (3)] of the first nitrogen ambient heat exchanger155, the nitrogen refrigerant passes through the compressor 160 in thecompressor-expander set 165, where the pressure of the nitrogen streamis increased to its greatest value [state (4)].

The nitrogen stream then passes through a second nitrogen ambient heatexchanger 170 [state (4) to state (5)]. Here the pressure again dropsslightly, the temperature, T, decreases, and entropy, s, is transferredout of the stream with the heat, all depicted in FIG. 2 b. More heat andentropy are transferred out of the nitrogen in a regenerative heatexchanger 175. The heat transferred out of the stream from state (5) tostate (6) enters the relatively lower temperature stream from state (8)to state (1). Note, again, in FIG. 2 b, the temperature, T, pressure, p,and entropy, s, all decrease in the regenerative heat exchanger 175 fromstate (5) to state (6).

The nitrogen stream is expanded from state (6) to state (7) in theexpander 180 of the compressor-expander set 165. As shown, the expander180 is outfitted with adjustable nozzles 210. At the discharge of theexpander 180 [state (7)], the temperature reaches its lowest value, asclearly seen in FIG. 2 b.

The subcooling heat exchanger 120 is encountered next, where thenitrogen stream passes from state (7) to state (8), gaining heat andentropy from the feedstock 100 stream. The temperature, T, of thenitrogen stream increases from state (7) to state (8), while thepressure, p, decreases due to friction.

Due to the relatively low temperature, T, of the nitrogen stream atstate (8), the stream can be used in the regenerative heat exchanger 175to reduce the temperature of the nitrogen stream from state (5) to state(6). The nitrogen stream entering the regenerative heat exchanger atstate (8) exits at state (1). The process from state (8) to state (1)involves an increase in heat, entropy, s, and temperature, T, and adecrease in pressure, p.

In FIG. 3, details of a control system are included, along with therefrigeration equipment shown in FIGS. 1 and 2 a. Recycle, or antisurge,valves 300, 305 are provided for the nitrogen recycle compressor 145 andthe compressor 160 of the compressor-expander set 165, respectively. Therecycle valves 300, 305 are used to vary the flow rate of the nitrogenthrough these compressors 145, 165. Surge avoidance in the nitrogenrecycle compressor 145 is effected by the manipulation of the firstrecycle valve 300. In the compressor of the compressor-expander set,antisurge control is effected through the manipulation of the secondrecycle valve 305.

In a typical LNG process, measured data are displayed, used for alarms,and for automatic control. Some of the transmitters used for the recyclecompressor 145 include: a flow transmitter, FT1 310, a suction pressuretransmitter, PT1 315, a discharge pressure transmitter, PT2 320, and arotational speed transmitter, SE1 480. The flow transmitter, FT1 310 isshown on the suction side of the recycle compressor 145 in FIG. 3, butthe present invention is not limited thereto.

Signals from the recycle compressor flow transmitter, FT1 310, suctionpressure transmitter, PT1 315, and discharge pressure transmitter, PT2320 are read into an antisurge control system, A/S PID 01 330, where anautomatic control algorithm, preferably a Proportional, Integral,Differential (PID) algorithm, is used to keep the recycle compressorfrom surging. The recycle compressor recycle valve 300 is manipulated bythe antisurge control system, A/S PID 01 330.

Some of the transmitters used for the compressor 160 of thecompressor-expander set 165 include: a flow transmitter, FT2 330, asuction pressure transmitter, FT3 335, a discharge pressure transmitter,FT4 340, and a rotational speed transmitter, SE2 345.

The flow transmitter, FT2 330 is shown on the discharge side of thecompressor 160 in FIG. 3, but the present invention is not limitedthereto.

Signals from the compressor-expander set compressor flow transmitter,FT2 330, suction pressure transmitter, PT3 335, discharge pressuretransmitter, PT4 340 are read into a second antisurge control system,A/S PID 02 350, where an automatic control algorithm is used to keep therecycle compressor from surging. The compressor-expander set compressorrecycle valve 305 is manipulated by the antisurge control system, A/SPID 02 350.

Note that redundant transmitters are not shown in FIG. 3. However,redundant transmitters are common in LNG processes.

A single compressor-expander set 165 is shown in each of FIGS. 1, 2 aand 3. Usually, however, multiple compressor-expander sets 165 areprovided and often arranged in parallel, serviced by a single nitrogenrecycle compressor 145 as shown in FIGS. 4 and 5. In FIGS. 4 and 5, four(4) compressor-expander sets 165, 410, 420, 430, are shown operating inparallel with one another. In FIG. 4 each compressor-expander set 165,410, 420, 430 is associated with its own subcooling heat exchanger 120.The plurality of subcooling heat exchangers 120 are arranged in serieson the LNG side, thus energy is removed from the product 100consecutively in each of the subcooling heat exchangers 120. All thecompressor-expander sets 165, 410, 420, 430 share a single subcoolingheat exchanger 120 in FIG. 5. The present invention is not limited toeither of these subcooling heat exchanger arrangements.

The sum of the mass flow rates of the nitrogen in all thecompressor-expander sets 165, 410, 420, 430 is determined using thesignals received from the flow transmitter, FT3 440, the pressuretransmitter, PT5 450, and the temperature transmitter, TT1 460.

The nitrogen is always superheated at the position of thesetransmitters, so the pressure and temperature are independentthermodynamic properties. Hence, the density, ρ, of the nitrogen gas maybe evaluated as ρ=ρ(p,T) and the mass flow rate, {dot over (m)}, isobtained by:{dot over (m)}=ρQ=A√{square root over (ρΔp)}where A is a constant associated with a differential pressure flow meterand Δp is the signal received from the flow transmitter 440. Theresulting mass flow rate, {dot over (m)}, is used in an automaticcontrol algorithm such as a PID loop as shown in FIG. 6 a.

Referring to FIG. 6 a, the raw signals from the transmitters 440, 450,460 may need to be scaled and an offset accommodated as shown in blocks610 to obtain actual values of pressure differential, Δp, pressure, p,and temperature, T.

The pressure, p, and temperature, T, values are used in the functionblock 620 for calculating the density, ρ, of the nitrogen as a functionof pressure, p, and temperature, T.

A first product block 630 is used to calculate the product of thepressure differential, Δp, and density, ρ. Then the square root of theproduct is found in the square root block 640.

A second product block 650 resolves the product of the square root ofthe product of the pressure differential, Δp, and density, ρ and theconstant, A 660. The result of the second product block 650 is thecalculated mass flow rate, {dot over (m)}, of the nitrogen. The massflow rate, {dot over (m)}, is used as the process variable in the massflow rate PID loop 670. The set point 680 is preferably provided by asupervisory or optimizing control system, but also may be entered by anoperator or field engineer. The output of the mass flow rate PID loop670 is directed to the adjustable nozzles 210 of one or more of theexpanders 180 in the compressor-expander sets 165, 410, 420, 430.

In FIG. 6 b, the transmitters associated with the main compressor: flowtransmitter, FT2 330, suction pressure transmitter PT3, 335, anddischarge pressure transmitter PT4, 340 may need to be scaled and anoffset accommodated as shown in blocks 610 to obtain actual values ofpressure differential, Δp, suction pressure, p_(s), and dischargepressure, p_(d).

In a first division block 615, the pressure differential, Δp, and thesuction pressure, p_(s), are combined to produce a dimensionless flowparameter denoted here as q². In a second division block 625, thedischarge pressure, p_(d), and the suction pressure, p_(s), are combinedto produce a dimensionless pressure ratio denoted here as R_(c).

The values of q² and R_(c) are combined to produce a measure ofproximity to a surge control line, S_(s), in a function block 635.

An identical process is carried out, using sensors and transmittersassociated with the balance compressor, to calculate a measure ofproximity to a surge control line, S_(s), for the balance compressor asindicated by the S_(s,balance) block 645. The two values of proximity tothe surge control line, S_(s,main), and S_(s,balance) are used in therespective antisurge control systems to protect these two compressorsfrom surge. These same values may be used by a load sharing andbalancing control system, whereby the rotational speeds of therespective compressors are manipulated via the expander adjustablenozzles 210. The overall performance of the combined system iscontrolled by the mass flow rate PID loop 670, which maintains the massflow rate at its set point.

A flow diagram outlining the critical speed avoidance algorithm is shownin FIG. 7 a. A rotational speed transmitter 470 is provided to each ofthe compressor-expander sets 165, 410, 420, 430. The critical speedavoidance control system receives a rotational speed signal, N, from therotational speed transmitter 470. A test is made in a first comparatorblock 705 to determine if the rotational speed signal, N, residesbetween the low boundary CS1 and the high boundary CS2 of the criticalspeed zone, and therefore indicates critical speed avoidance is needed.If the result of the first comparator block 705 is false, a timer is setto t=0 in timer set block 710 and the rotational speed, N, continues tobe monitored.

The first instance the result of the first comparator block 705 is true,the timer is initiated 715. Any time the result of the first comparatorblock 705 is true, the time reported by the timer is compared to apredetermined maximum time, t_(max), the compressor-expander set 410will be permitted to operate in the critical speed zone. This operationis carried out in block 720. If the maximum time, t_(max), time has beenexceeded, the control system will initiate an orderly shutdown of thecompressor expander set 410 as shown in shutdown block 725. As long asthe maximum time limit, t_(max), has not been exceeded, a determinationis made in a second comparator block 730 whether the compressor-expanderset 410 rotational speed, N, is less than the minimum operationalspeed—and is therefore in startup mode. If the result of the secondcomparator block 730 is true, the recycle valve 305 associated with thecompressor 160 in the compressor-expander set 410 is ramped closed 735at a predetermined ramp rate 740 until either of the results of thefirst comparator block 705 or the second comparator block 730 is false.It should be noted that, typically, the recycle valve 305 is held openon startup.

If the result of the second comparator block 730 is false when theresult of the first comparator block 705 is true, it is concluded thecompressor-expander set 410 rotational speed, N, is within the normaloperating range. In this case, the recycle valve 305 is ramped open 745at a predetermined ramp rate 750 until the result of the firstcomparator block 705 is false.

The shutdown block 725 is expanded in a representative shutdownprocedure in FIG. 7 b. Such a procedure is used regardless of the reasonfor the shutdown. As those of ordinary skill in this art are well aware,a shutdown may be planned in advance, or it may be an emergency shutdowndue to a sensed condition demanding immediate shutdown.

Referring now to FIG. 7 b, to keep the nitrogen recycle compressor 145from surging upon a trip of one of the compressor-expander sets 165,410, 420, 430, a feedforward signal is provided to the nitrogen recyclecompressor's control system, A/S PID 01, 330.

This step is shown in feedforward block 755. The nitrogen recyclecompressor's control system 330 will act to increase the opening of thenitrogen recycle compressor's antisurge valve 300, as shown in theresultant block 760, when it receives this feedforward signal. Theincrease in opening may be a predetermined, fixed amount, or theincrease may be calculated based on operating parameters of the nitrogenrecycle compressor 145 and/or the tripped compressor-expander set 165,410, 420, 430.

Other steps in the shutdown procedure, not necessarily in the order inwhich they will be carried out include: opening the compressor-expanderset compressor 160, 810, 820 recycle valve as shown in the recycle block765; closing the expander adjustable nozzles 210 as indicated in thenozzle block 770; closing the expander shutdown valve 850 or 860 asillustrated in the shutdown valve bock 775; and alarming the operator ofthe shutdown 780. The order in which these steps are carried out dependson the system, rates of actuation, and personal preference.

In FIG. 8, a preferred piping system is illustrated. In this pipingarrangement, the compressors 810, 820 within the main and balancecompressor-expander sets, respectively, are plumbed in parallel. Theexpanders 830, 840, on the other hand, are not in parallel with oneanother. Rather, the main expander 830 feeds the subcooling heatexchanger 120, while the exhaust from the balance expander 840 iscombined with the discharge of the subcooling heat exchanger 120, andthe entire flow is used as the cold fluid in the regenerator 175. Thispiping arrangement is presented to provide a full disclosure of thesystems on which the present invention may be used. However, the methodand apparatus of the instant invention is unaffected by the known pipingvariations shown in FIGS. 1, 2 a, 3-5, and 8.

Expander shutdown valves 850, 860 are provided for shutting down themain and balance compressor-expander sets.

Performance maps for the main compressor 810 are shown in FIGS. 9 and10. Performance maps for the balance compressor 820 are shown in FIGS.11 and 12. As those skilled in this art know, for a given rotationalspeed, N, the shaft power required by a centrifugal compressor 160, 810,820 often has a positive slope as shown in FIGS. 10 and 12. Frequently,as especially seen in FIG. 12, at high flow rates, the shaft power curvefor a constant rotational speed, N, may have a negative slope. Eitherway, the shaft power required by the centrifugal compressors 160, 810,820 changes with flow rate. Therefore, the shaft power varies with theopening of the recycle valve 305. The speed is governed by the equation:

$\begin{matrix}{{\frac{1}{2}I\frac{\mathbb{d}N^{2}}{\mathbb{d}t}} = {{\overset{.}{W}}_{i\; n} - {\overset{.}{W}}_{out}}} & (1)\end{matrix}$where I is the moment of inertia for the compressor-expander set 410, Nis the rotational speed of the compressor-expander set 410, {dot over(W)}_(in) is the shaft power supplied to the compressor 160, 810, 820,and {dot over (W)}_(out) is the shaft power required by the compressor160, 810, 820. Opening the recycle valve 305 changes {dot over(W)}_(out), and hence the rotational speed, N. Therefore, changing theopening of the recycle valve 305 may be used to move the rotationalspeed, N, of the compressor-expander set 410 out of a critical speedzone.

The above embodiment is the preferred embodiment, but this invention isnot limited thereto. It is, therefore, apparent that many modificationsand variations of the present invention are possible in light of theabove teachings. It is, therefore, to be understood that within thescope of the appended claims, the invention may be practiced otherwisethan as specifically described.

The invention claimed is:
 1. A method of surge avoidance for a recyclecompressor within a gas refrigeration system, the gas refrigerationsystem comprising a compressor-expander set in fluid communication andconnected in series with the recycle compressor, a compressor-expanderset control system, a recycle compressor control system, and a recyclecompressor antisurge valve, the method comprising: (a) causing thecompressor-expander set to shutdown; (b) sending a feedforward signalfrom the compressor-expander control system to the recycle compressorcontrol system indicating a shutdown of the compressor-expander set; and(c) increasing an opening of the recycle compressor antisurge valve uponreception of the feedforward signal by the recycle compressor controlsystem.
 2. The method of claim 1 wherein increasing the opening of therecycle compressor antisurge valve comprises increasing the opening ofthe recycle compressor antisurge valve a predetermined amount.
 3. Anapparatus for surge avoidance for a recycle compressor comprising: (a) agas refrigeration system with which the recycle compressor is in fluidcommunication; (b) a compressor-expander set in fluid communication andconnected in series with the recycle compressor; (c) acompressor-expander set control system; (d) a recycle compressor controlsystem; (e) a recycle compressor antisurge valve; (f) a shutdown signalfrom the compressor-expander set to signal the compressor-expander setto shutdown; (g) a feedforward signal from the compressor-expandercontrol system to the recycle compressor control system indicating ashutdown of the compressor-expander set; and (h) a signal from therecycle compressor's control system to signal an increase of opening ofthe recycle compressor antisurge valve upon reception of the feedforwardsignal by the recycle compressor control system.